It has been recently proposed that a particulate filter for capturing particulates in exhaust gas is incorporated in an exhaust pipe and a selective reduction catalyst capable of selectively reacting NOx with ammonia even in the presence of oxygen is arranged downstream of the particulate filter, urea water as reducing agent being added between the selective reduction catalyst and the particulate filter, thereby attaining reduction of both the particulates and NOx.
Such addition of the urea water to the selective reduction catalyst is conducted between the particulate filter and the selective reduction catalyst. Thus, in order to ensure sufficient reaction time for pyrolysis of the urea water added to the exhaust gas into ammonia and carbon dioxide gas, it is necessary to prolong a distance between a urea water added position and the selective reduction catalyst. However, such arrangement of the particulate filter and the selective reduction catalyst substantially spaced apart from each other will extremely impair the mountability on a vehicle.
In order to overcome this, a compact exhaust emission control device as shown in FIGS. 1 and 2 has been proposed by the inventor as Japanese patent application No. 2007-29923. In the exhaust emission control device of the prior application shown, incorporated in an exhaust pipe 4 through which exhaust gas 3 flows from a diesel engine 1 via an exhaust manifold 2 is a particulate filter 5 housed in a casing 7 to capture particles in the exhaust gas 3; arranged downstream of and in parallel with the particulate filter 5 and housed in a casing 8 is a selective reduction catalyst 6 having a property capable of selectively reacting NOx with ammonia even in the presence of oxygen. An exit-side end of the particulate filter 5 is connected to an entry-side end of the selective reduction catalyst 6 through an S-shaped communication passage 9 such that the exhaust gas 3 discharged through the exit-side end of the particulate filter 5 is reversely curved back into the entry-side end of the adjacent selective reduction catalyst 6.
As shown in FIG. 2 which shows substantial parts in enlarged scale, the communication passage 9 is the S-shaped structure comprising a gas gathering chamber 9A which encircles the exit-side end of the particulate filter 5 to gather the exhaust gas 3 through substantially perpendicular turnabout of the gas just discharged from the exit-side end of the particulate filter 5, a mixing pipe 9B which extracts the gathered exhaust gas 3 from the chamber 9A in a direction reverse to that of the exhaust gas flow in the filter 5 and which is provided with an injector 11 for urea water addition (urea water addition means) intermediately of the mixing pipe and a gas dispersing chamber 9C which encircles the entry-side end of the selective reduction catalyst 6 so as to disperse the gas 3 guided by the mixing pipe 9B through substantially perpendicular turnabout into the entry-side end of the selective reduction catalyst 6.
Arranged in the casing 7 and in front of the particulate filter 5 is an oxidation catalyst 14 for oxidization treatment of unburned fuel in the exhaust gas 3. Arranged in the casing 8 and behind the selective reduction catalyst 6 is an ammonia reducing catalyst 15 for oxidization treatment of surplus ammonia.
With the exhaust emission control device thus constructed, particulates in the exhaust gas 3 are captured by the particulate filter 5. The urea water is added intermediately of the mixing pipe 9B and downstream of the filter into the exhaust gas 3 by the injector 11 and is pyrolyzed into ammonia and carbon dioxide gas, so that NOx in the exhaust gas 3 is satisfactorily reduced and depurated by the ammonia on the selective reduction catalyst 6. As a result, both the particulates and NOx, in the exhaust gas 3 are reduced.
In this case, the exhaust gas 3 discharged from the exit-side end of the particulate filter 5 is reversely curved back by the communication passage 9 into the entry-side end of the adjacent selective reduction catalyst 6. As a result, enough reaction time is ensured for production of ammonia from the urea water since a long distance between the urea water added position intermediately of the communication passage 9 and the selective reduction catalyst 6 is ensured and the flow of the exhaust gas 3 becomes turbulent due to the reversed curving to facilitate mixing of the urea water with the exhaust gas 3.
Moreover, the particulate filter 5 and selective reduction catalyst 6 are arranged in parallel with each other and the communication passage 9 is arranged between and along the particulate filter 5 and selective reduction catalyst 6, so that the whole structure becomes compact in size to substantially improve its mountability on a vehicle.
However, as illustrated in FIGS. 1 and 2, addition of the urea water to the selective reduction catalyst 6 at between the particulate filter 5 and the selective reduction catalyst 6 is conducted in such a manner that a boss 10 is provided intermediately of and branched obliquely from the mixing pipe 9B to be directed upstream, the injector 11 for addition of urea water being fitted into the boss 10 from outside of the mixing pipe 9B, urea water being added with the injector 11 being protected so as not to be directly exposed to the flow of the hot exhaust gas 3. Thus, when the flow rate of the exhaust gas 3 is increased as shown in FIG. 3, the energetic flow of the exhaust gas 3 may urge the urea water added through the injector 11 to be biased against and along an inner wall of the mixing pipe 9B, resulting in failure of satisfactorily dispersion of the urea water.
In order to overcome this, the inventor came to devise out that, as shown in FIGS. 4 and 5, a downstream end 9a of the gas gathering chamber 9A is changed in its connection with the entry-side end of the mixing pipe 9B to surround the latter in a properly spaced manner with an open edge of the entry-side end being closed. The entry-side end of the mixing pipe 9B is formed with first and second openings 12 and 13 and is provided with first, second and third partitions 16, 17 and 18, so that the exhaust gas 3 from the particulate filter 5 swirls in one direction around the entry-side end of the mixing pipe 9B and guided tangentially into the same through the first and second openings 12 and 13; the injector 11 is fitted at and axially of the entry-side end of the mixing pipe 9B for addition of the urea water by the injector 11 axially of the entry-side end of the mixing pipe 9B (this application has been filed and allotted Japanese patent application No. 2007-56963).
It is contemplated that, with such construction, swirling flow is effectively formed in the mixing pipe B by inflow of the exhaust gas 3 through the first and second openings 12 and 13; the urea water is added by the injector 11 axially into the entry-side end of the mixing pipe 9B where the swirling flow is most energetic, so that the added urea water is satisfactorily dispersed into the exhaust gas 3 by the swirling flow to substantially enhance the mixing with the exhaust gas 3.
As a prior art literature pertinent to the invention, there exists, for example, the following Patent Literature 1.    [Patent Literature 1] JP 2005-155404A